Method of forming micro-sensor thin-film anemometer

ABSTRACT

A device for measuring turbulence in high-speed flows is provided which includes a micro-sensor thin-film probe. The probe is formed from a single crystal of aluminum oxide having a 14° half-wedge shaped portion. The tip of the half-wedge is rounded and has a thin-film sensor attached along the stagnation line. The bottom surface of the half-wedge is tilted upward to relieve shock induced disturbances created by the curved tip of the half-wedge. The sensor is applied using a microphotolithography technique.

ORIGIN OF THE INVENTION

The invention described herein was made jointly in the performance ofwork under NASA Grant No. NAG-1-1400 with Syracuse University and agraduate student, and employees of the United States Government. Inaccordance with 35 U.S.C. 202, the grantee elected not to retain title.

This is a divisional of application Ser. No. 08/361,601 filed on Nov.21, 1994, U.S. Pat. No. 5,576,488.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a device and method formeasuring turbulence in high-speed flows, and more particularly to amicrosensor thin-film probe capable of measuring turbulence inhypersonic flows.

2. Description of the Related Art

Turbulence measurements in high-speed flows have historically beenobtained by hot-wire anemometry. However, high stagnation temperatures,high dynamic pressures, and flow contaminants severely limit the life ofhot-wire elements in hypersonic flow. Nonintrusive measurementtechniques such as laser-Doppler velocimetry and particle-imagevelocimetry also are limited when applied to hypersonic flow. Inparticular, data-rate limitations and difficulties in flow seedingpresent the most significant obstacles to their accurate application inhigh-speed flows. An alternative to conventional hot-wire anemometry ishot-film anemometry, with a thin metallic film deposited along thestagnation line of a rigid, dielectric substrate, thus increasingmechanical strength.

Hot-film probes incorporating various combinations of materials andconstruction techniques have displayed excellent durability and moderatefrequency response characteristics in the few high-speed and hightemperature flows in which they have been tested.

Ling and Hubbard introduced the thin-film probe as aresistance-temperature transducer to measure turbulent fluctuations inflowfields in which hot-wires could not survive (Ling, S. C. andHubbard, P. G. 1956 "The Hot-Film Anemometer: A New Device for FluidMechanics Research", J. Aeronaut. Sci. 23, 890). This probe consisted ofa thin layer of platinum fused to a glass substrate. The main body ofthe probe consisted of a 4.0 mm diameter Pyrex rod with two 32-gaugeplatinum lead wires (2.0 mm apart) embedded within the core. The rod wasground down into a 8° wedge, tipped by a 30° wedge. Fused on the wedgetip was a thin platinum film sensor (1.0 mm×0.2 mm) with a nominal coldresistance of 20Ω. The ends of the sensor were attached to the exposedlead wires by thick platinum overplatings. It was tested at hightemperatures (1100° F.) without detectable changes in thermoelectricproperties. Experiments indicated a frequency response of 100 kHz at aflow velocity of 1000 ft/sec.

Later, Seiner used commercial hot-film probes to investigate high-speed,cold jet flows (Mach 0.5-2.0), (Seiner, J. M. 1983 "The Wedge Hot-FilmAnemometer in Supersonic Flow", NASA Technical Paper 2134). The probeconsisted of a thin film of nickel sputtered on a 4° semivetrex wedge ofquartz substrate. A protective coating of quartz (0.5-2.0 μm) wassputtered over the nickel (1.0 mm×0.2 mm) for electrical isolation andprotection from particles. A maximum frequency response of 130 kHz forthe 1:1 balanced CTA bridge was realized via the square-wave injectiontest, and was found to be inversely proportional to the thickness of theprotective coating.

More recently, Demetriades and Anders presented a report on the ongoingdevelopment of a constant-current probe for use inhigh-temperature/hypervelocity flows (Demetriades, A. and Anders, S. G.1990 "Characteristics of Hot-Film Anemometers for Use in HypersonicFlows", AIAA Journal 28, 2003). The design consists of a platinum sensor(0.5 mm×1.8 mm) painted along the stagnation line of a wedge-shapedglaze bead positioned at the tip of a twin-bore alumina tube (10.0cm×0.25 cm). Results from temperature endurance testing (temperaturecycling to 1400° F.) demonstrated the excellent electrical stabilitycharacteristics of the probe. These probes were also run for hours inhigh-temperature, high dynamic-pressure environments (Mach 8.0, 800° F.and 20.7 kPa) without failure, further confirming the durabilitycharacteristics. Presently, there is no experimental data available fromDemetriades documenting the frequency response characteristics of thisprobe. However, painted sensors contain cross-sectional non-uniformitieswhich lead to "hot-spots" and sensor failures. In addition, the largesensor surface area used by Seiner and Demetriades limits the spatialresolution of the measurement technique.

The frequency response characteristics of the existing hot-film probesare inadequate to resolve the full turbulent spectrum for hypersonicflows. The "dual swept-surface" wedge designs used by Seiner are a poorapproximation of stagnation-line heat-transfer, since the sensor extends0.1 mm away from the tip on both sides of the wedge. In addition to thisfundamental problem, Seiner found that the probe displayed poordirectional behavior and was Mach-number sensitive. Seiner hypothesizedthat these problems were associated with the 40° wedge geometry andrecommended examination of a larger semi-vertex angle or a wedge fittedwith a rounded nose. Finally, this design is unacceptable formeasurements in wallbounded shear flows, as shocks emanating from thebottom of the probe would disturb the flow.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a device andmethod for measuring turbulence in high-speed flows such as hypersonicflows.

A further object of the present invention is to measure turbulence inhigh-speed flows with a device and method having fast response anddurability in the severe environment of hypersonic airflows.

The foregoing objects are achieved by providing an anemometer having amicro-sensor thin-film probe. The probe is a half-wedge formed from asingle crystal of aluminum oxide (i.e., sapphire) and is configured toprevent the wedge from generating a detached shock through much of ahypersonic boundary layer. The tip of the sapphire probe is rounded tominimize flow disturbance and contains an iridium sensor formed on therounded tip of the probe along the stagnation line.

The iridium sensor is formed by first depositing a layer of copper overthe sapphire substrate in the area where the sensor is to be located. Alayer of photoresist is then deposited over the copper and dried. Acontact print of the sensor shape is then made into the photoresist byexposing the photoresist to ultraviolet light. The photoresist is thendeveloped leaving an opening to the copper layer corresponding to thesensor shape. The exposed copper is etched to produce an opening throughthe copper to the sapphire substrate corresponding to the sensor shape.The photoresist is removed and niobium is deposited on the exposedsapphire substrate by electron beam vapor deposition. Without breakingthe vacuum, iridium is deposited by electron beam vapor deposition ontothe niobium layer. The copper is then removed with an etchant and theprobe containing the sensor is annealed in a hard vacuum atapproximately 1000° C. to stabilize the resistance of the sensor. Oncethe sensors are formed, they are connected to leads which connect to acoaxial cable. The coaxial cable connects to the driving circuit of theanemometer.

Sensors formed according to this method show a significant improvementin frequency response due to the lower thermal inertia of the sensorcompared to conventional 5.0 μm diameter hot wire and existing hotfilms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view of the thin-film probe of the present invention.

FIG. 1B is a side view of the tip of the thin-film probe depicted inFIG. 1A.

FIG. 1C is an illustration of the angle of a portion of the probedepicted in FIGS. 1A and 1B.

FIG. 2 is a drawing of the sensor region of the thin-film probe of thepresent invention.

FIG. 3A is a drawing of the sensor pattern formed through thephotoresist and copper layers.

FIG. 3B is a drawing of the niobium and iridium layers of the sensordeposited through the copper layer.

FIG. 3C is a drawing of the sensor formed on the sapphire substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, the probe 10 geometry consists of a straightportion leading to an approximately 14° half-wedge, 1/2 inch long×1/8inch wide, diamond tooled out of a single crystal aluminum oxide (i.e.,sapphire). The use of sapphire for the substrate material enables theprobe to withstand severe temperatures without mechanical failure or asignificant change in dielectric properties. The capability of sapphireto be machined and polished to a finish less than one micro-inch(r.m.s.) coupled with the microphotolithographic technique allows for asignificant reduction in sensor size over existing hot-films, thusimproving the spatial resolution of the probe. In addition, thesubstrate material was selected because it has a low thermalconductivity and a coefficient of thermal expansion that closely matchesthe iridium and niobium layers, thus reducing the risk of sensordetachment during thermal expansion. The 14° half-angle representsθ_(crit) for an air flow with a freestream Mach number slightly below1.6, thus preventing the wedge from generating a detached shock throughmuch of a hypersonic boundary layer. This angle can be modified forapplication to specific airflow conditions. The half-wedge geometry ispreferred in order to minimize flow disturbance and allow for near-wallmeasurements in boundary layers. Referring to FIG. 1B, the bottomportion of the wedge is angled upward at 3° in an effort to relieveshock induced disturbances created by the finite curvature of the nose.This will lessen any boundary-layer disturbance that can propagateupstream, making for more accurate measurements. This angle of tilt canbe modified to suit the application and different wedge tip radii. Thewedge tip 20 is rounded to a radius of curvature of approximately 0.2mm, and is then diamond polished to a r.m.s. surface finish of 1microinch or less to prepare the surface for deposition of the sensor30. The radius of curvature of the wedge tip 20 should be as small aspossible to minimize flow disturbance while still accommodating thesensor. The rounded-nose, wedge tip geometry offers benefits over the"dual swept-surface" probes. Since the sensor is very narrow, "true"stagnation point heat-transfer will occur: existing heat transfer datafor heated cylinders in turbulent cross-flow shows a negligible decreasein local Nusselt number for angles less than 5° from the stagnationline. It is thus possible to approximate the rounded nose as a cylinderin cross-flow, for which a considerable amount of data exists in allMach number regimes. The onset of Mach number independence may also bereduced, as a normal shock will occur locally in front of the stagnationline, rather than a weaker oblique shock in front of a wedge.

A "dog-bone" shaped micro-sensor (approx. 2000 Å×12.5 μm×0.25 mm) 30 ofiridium is deposited along the stagnation line 40 of the substrate usinga microphotolithography technique, as described below. The "dog-bone"shape of the sensor results in reducing failures at the junction of thelead wire with the sensor because the "dog-bone" is wider at the ends sothe actual junction doesn't become too hot. Additional turbulencemeasurements may be made from additional sensors located off thestagnation line.

To form the sensor 30, an approximately 4000-5000 Å thick layer ofcopper 50 is deposited by sputtering or vapor deposition onto the sensorarea of the substrate 60. Although copper is preferred, other materialssuch as Cr, Ag and Ni can be used which can be selectively etched. A 2.0μm thick layer of positive-phase photoresist 70 is then spun onto thecopper layer 50 and baked. A negative of the "dog-bone" shaped sensorpattern is contact printed into the photoresist layer 70 by exposing thephotoresist to ultraviolet light, preferably a highly collimatedmercury-vapor light source. The photoresist is developed, leaving anopening 80 through to the copper layer 50 with a shape corresponding tothe shape of the sensor. The copper is then etched with a suitableetchant such as ammonium phersulphate to produce an opening in thecopper layer shaped like the sensor pattern. This opening extends to thesapphire substrate 60. The photoresist 70 is removed and a 150-200 Åthick layer of niobium 90 is then sputter deposited or vapor depositedonto the sapphire substrate 60 through the opening in the copper layer50. Although niobium is preferred, other materials such as chromium canbe used as long as the material selected is stable at high temperaturesand is essentially nonreactive with adjacent layers. Deposition byelectron beam vapor deposition is preferred because the vapor flux tendsto be directional. The thickness of the niobium layer 90 can vary, butmust be thick enough to provide an adhesive base and to ensure that theniobium is contiguous throughout the layer. Without breaking vacuum, anapproximately 2000 Å layer of iridium 100 is deposited on the niobiumlayer 90 by sputtering or vapor deposition, although electron bean vapordeposition is preferred. The thickness of the iridium can be varied toproduce a sensor having a desired resistance. Although iridium ispreferred, other materials such as platinum can be used as long as thematerial selected is stable at high temperatures, has a reasonablethermal coefficient of resistance in the temperature range of interest,and is essentially nonreactive with the underlying layer. The copperlayer 50 is removed with an etchant such as ammonium phersulphate andthe substrate 60 containing the sensor 30 is annealed in a hard vacuumat approximately 1000° C. to stabilize the resistance of the sensor. Thehard vacuum is required because iridium slowly forms volatile oxides attemperatures above approximately 900° C.

The sensor 30 is then connected at each end to a corresponding lead 110which is electrically connected to the driving circuit of the anemometer(not shown). Any suitable leads can be used as long as the leads do notprotrude above the surface of the sapphire substrate enough to affectthe airflow. For example, organometallic leads could be painted on, orthe leads could be deposited by sputtering or vapor deposition inchannels formed in the substrate. The leads can be any electricallyconductive material having suitable resistance and stability at theoperating temperature of the anemometer.

The microphotolithographic technique allows for a significant reductionin sensor size over previous designs, thus improving the spatialresolution of the probe. The thermal inertia of the sensor is two ordersof magnitude smaller than that of a conventional 5.0 μm diameter hotwire and existing hot films. Therefore, a significant improvement infrequency response is expected. Preliminary results indicate a frequencyresponse of 800 kHz via square-wave injection.

Although the present invention has been described in detail with respectto certain preferred embodiments thereof, it is understood by those ofskill in the art that variations and modifications in this detail may bemade without any departure from the spirit and scope of the presentinvention, as defined in the hereto-appended claims.

What is claimed is:
 1. A method forming a thin-film probe, comprising the steps of:providing a substrate of single crystal of aluminum oxide; forming the substrate into a half-wedge shape having a tip with a radius of curvature less than 5 mm and a r.m.s. surface finish of not more than 1 microinch; forming a thin-film sensor on the tip of the substrate, wherein the steps of forming the thin-film further comprises: depositing a layer of copper on the curved tip of the substrate; depositing a layer of positive-phase photoresist onto the copper layer; drying the photoresist layer; contact printing a negative of a sensor shape into the photoresist layer along the stagnation line of the tip of the substrate using ultraviolet light; developing the photoresist to leave a sensor-shaped opening to the copper layer; etching the copper layer to produce a corresponding sensor-shaped opening to the substrate; removing the layer of photoresist; depositing a layer of niobium onto the substrate through the sensor-shaped opening in the copper layer; depositing a layer of iridium onto the niobium layer through the sensor-shaped opening in the copper layer; removing the copper layer; and annealing the substrate having the thin-film sensor in a hard vacuum at approximately 1000° C. to stabilize the resistance of the thin-film sensor.
 2. The method of claim 1, wherein the thickness of the copper layer is approximately 5000Å or less.
 3. The method of claim 2, wherein the ultraviolet light is a highly collimated mercury-vapor light source.
 4. The method of claim 3, wherein the niobium layer is approximately 200Å or less.
 5. The method of claim 4, wherein the iridium layer is approximately 2000Å or less.
 6. The method of claim 5, wherein the annealing temperature is approximately 1000° C. and the vacuum is at least approximately 10⁻⁵ Torr. 